Direct Heat Flux Research Articles

Electrocaloric effect in chemically modified barium titanate ferroelectric ceramics

Electrocaloric (EC) refrigeration offers superior energy-conversion efficiency, miniaturization, and environmental benefits compared to compression refrigeration. However, its practical application is limited by the challenge of aligning the adiabatic temperature change (ΔT) with the operational temperature range. In this study, we have tailored the EC characteristics of BaTiO3 (BT)-based Ba(Ti0.9Sn0.1)O3 (BTS) ferroelectric ceramics using Bi(Mg0.5Ti0.5)O3 (BMT). We provide a comprehensive analysis of the microstructure, electrical properties, and EC behavior of the (1–x)BTS-xBMT system. Our results indicate that the incorporation of BMT establishes a broad platform in the dielectric spectrum while maintaining high polarization. This improvement potentially increases the electrocaloric effect (ECE) and expands the operating temperature range. Direct heat flux measurements reveal that the x = 0.02 composition achieves a maximum ΔT (ΔTmax) of 0.41 K with a temperature span (Tspan) of 68 °C under an intermediate electric field of 50 kV/cm. Moreover, the x = 0.01 sample exhibits a room-temperature ΔT of 0.33 K and relatively good temperature stability within 30–130 °C. These findings indicate that chemical modification methods have the potential to optimize the cooling capacity of refrigerants.

Comparison between Direct and Indirect Heat Flux Measurement Techniques: Preliminary Laboratory Tests

Direct and indirect approaches can be employed for estimating the heat flow through components in different application fields. In the building sector, the thermometric method is often applied by professionals for thermal transmittance evaluations. However, miscalculations can derive from inaccurate total heat transfer coefficients, and a consensus regarding the appropriate value to employ remains to be determined. Here, an apparatus was realized for laboratory tests and heat flux measurements were performed following direct and indirect approaches. Data acquired through a common heat flow sensor were compared with those computed through a post-processing based on radiative and convective estimations. The results were affected by the specific correlation adopted for computing the convective coefficients, with the percentage differences ranging from −9.8% to −0.4%. New measurement systems could be designed for automatically computing heat fluxes through indirect approaches, thus providing alternative solutions in the panorama of non-destructive tests for building energy diagnosis.

Heat flux measurement approach for an enhanced thermometric method: preliminary tests

In situ tests are suitable to confirm the real thermal performance of building components, and several significant on-site measurement techniques have been studied in literature. However, among them the Thermometric (THM) method has been poorly examined by the scientific community, thus having opportunities for improvement, being a quite a new and non-standardized technique. The theory behind this technique is the Newton’s law of cooling and the main issue is associated to the heat transfer coefficient for which there is no agreement about the value to use. Here, a simple experimental apparatus characterized by a vertical heated sample, suitably thermally insulated was realized. Sensors were installed and direct heat flux measurements through a heat flux plate were performed and compared with (i) the heat flows obtained through the THM method (test conducted using the internal heat transfer coefficient recommended by the ISO 6946) and (ii) the heat fluxes obtained through the proposal of an enhanced THM method based on dimensionless groups analysis, thus requiring data processing based on convective and radiative components.

TwoPhaseFlow: A Framework for Developing Two Phase Flow Solvers in OpenFOAM

We present a new OpenFOAM based open-source framework, TwoPhaseFlow, enabling fast implementation and testing of new phase change and surface tension force models for two-phase flows including interfacial heat and mass transfer. Capitalizing on the runtime-selection mechanism in OpenFOAM, the new models can easily be selected and benchmarked against analytical solutions and existing models. The framework currently includes the following three interface curvature calculation methods for surface tension: 1) the height function method, 2) the parabolic fit method and 3) the reconstructed distance function method. As for phase change, two models are available: 1) Interface heat resistance and 2) direct heat flux. These can be combined in three solvers: 1) InterFlow for isothermal, incompressible two-phase flow, 2) compressibleInterFlow for compressible, non-isothermal two-phase flow and 3) multiRegionPhaseChangeFlow for compressible, non-isothermal two-phase flow with conjugated heat transfer. By design, addition of new models and solvers is straightforward and users are encouraged to contribute their specific models, solvers, and validation cases to the library.

Separating Direct Heat Flux Forcing and Freshwater Feedback on AMOC Change Under Global Warming

AbstractThe Atlantic meridional overturning circulation (AMOC) is predicted to weaken under global warming. Whether it is caused by heat flux or freshwater flux is under debate. Here we separate these two processes in changing the AMOC under global warming. The simulated AMOC is weakened during the first 600 years and then gradually recovered to its initial state, with heat flux and freshwater feedback dominating at different timescales. Global warming immediately puts freshwater into the Southern Ocean, which triggers the initial AMOC weakening via altering surface temperature. Concurrently, the extensive heat into the ocean surface increases the temperature over the subpolar North Atlantic, reducing the deep convection and thus the AMOC in the subsequent 50–150 years. Meanwhile, the Arctic sea ice melt leads to the AMOC shutdown. Subsequently, the salinity accumulation in the subtropical North Atlantic propagating northward to restart the North Atlantic deep convection is responsible for the AMOC recovery.

Parametric study of the energy potential of a building’s envelope with integrated energy-active elements

Building structures with integrated energy-active elements (BSIEAE) present a progressive alternative for building construction with multifunctional energy functions. The aim was to determine the energy potential of a building envelope with integrated energy-active elements in the function of direct-heating, semi-accumulation and accumulation of large-area radiant heating. The research methodology consists in an analysis of building structures with energy-active elements, creation of mathematical-physical models based on the simplified definition of heat and mass transfer in radiant large-area heating, and a parametric study of the energy potential of individual variants of technical solutions. The results indicate that the increase in heat loss due to the location of the tubes in the structure closer to the exterior is negligible for Variant II, semi-accumulation heating, and Variant III, accumulation heating, as compared to Variant I, direct heating, it is below 1 % of the total delivered heat flux. The direct heat flux to the heated room is 89.17 %, 73.36 %, and 58.46 % of the total heat flux for Variant I, Variant II and Variant III, respectively. For Variant II and Variant III, the heat storage accounts for 14.84 %, and 29.86 % of the total heat flux, respectively. Variants II and III appear to be promising in terms of heat/cool accumulation with an assumption of lower energy demand (at least 10 %) than for low inertia walls. We plan to extend these simplified parametric studies with dynamic computer simulations to optimise the design and composition of the panels with integrated energy-active elements.

Thermo‐Electric Properties of Poly(lactic) Acid Filled with Carbon‐Based Particles: Experimental and Simulation Study

AbstractPolymer composites filled with high thermal and electrical conductivity nanofillers show enhanced thermo‐electric properties which encourage their use in heat transfer applications. In the present study, nanocomposites based on polylactic acid (PLA) containing up to 9 wt% of multi‐walled carbon nanotubes (trade name: N7000) and graphene nanoplates (trade name: TNIGNP) are produced via melt compounding and then morphologically, electrically, and thermally investigated. The results are correlated to the different characteristic of the fillers and their interaction with the PLA. At the highest investigated filler concentration, an electrical conductivity of about 2 S m–1 and a thermal conductivity of 0.725 W m–1 K–1 are measured respectively for nanocomposites based on N7000 and TNIGNP, which are decisively higher than the values measured for unfilled PLA (5.9 ×∙10–2 S m–1 and 0.205 W m–1 K–1). Moreover, multiphysics simulations are performed on the best performing nanocomposites for evaluating their thermo‐electric properties when an electrical heating or a direct heat flux are applied.

Bi-objective topology optimization for direct current concentration and heat flux cloaking using adaptive weighting method

Bi-objective topology optimization for direct current concentration and heat flux cloaking using adaptive weighting method

Temperature guided behavioral transitions in confined helium: Gas-wall interaction effects on dynamics and transport in the cryogenic limit

The low-temperature dynamics and transport of a real gas He, in confined condition are studied employing molecular dynamics simulations. Characterization of the gaseous properties at varying temperatures (T) in the range 30 - 150 K, is carried out employing velocity autocorrelation function (VACF), mean squared displacement (MSD), and the transport properties such as, self-diffusion coefficient (D) and thermal conductivity (λ) are determined. In the confined system, gas-wall interactions are found to dictate the above properties, and near-wall accumulation of molecules is observed. Further, depending upon T, the effect of wall-mediated forces is found on the distant gas particles. At very short timescales (t* < 0.1), gas-wall interactions result in faster decay of VACF and shorter ballistic phase in MSD along the confining direction, which are most prominent at higher T, compared to those for the unconfined directions. Whereas, at short timescales (t*∼ 0.5) non-thermal gas-wall collisions are observed, leading to a minimum in VACF along the confining direction depending upon T. At longer timescales (t* > 1), gas-wall particle collisions lead to the sub-diffusive behavior of the confined gas along the confinement, which is most prominent at higher T. The results are compared with those for the bulk He to quantify the confining effect on the gas. Reasonably good agreement of the bulk transport properties with numerically calculated quantum mechanical results for LJ potential and existing results from literature (experimental and quantum mechanical) has been found. In the confined system, the parallel self-diffusion coefficient and thermal conductivity (D∥ and λ∥) are found to be close to the bulk values. Additionally, λ is estimated from direct heat flux measurements using the Green-Kubo (G-K) formalism and non-equilibrium method for the confined system, and the results are compared. The findings of this work have far-reaching consequences in investigating complex systems.

Direct heat flux sensing for window shading control in passive cooling systems

Greenhouse gas emissions from space cooling are expected to double globally by 2050, giving urgency to the development of low-carbon cooling methods. Solar heat gain through windows is a leading contributor to cooling loads, and accordingly, operable shading strategies have been established that respond to numerous environmental parameters, including solar radiation, illumination, time of day, and indoor and outdoor air temperature. While these have shown excellent performance, optimal setpoints are typically specific to climates, seasons, and spaces, preventing development of controls with consistent effectiveness across varying conditions. Here we investigate an alternative parameter, window surface heat flux, in which a universal setpoint of 0W/m2 identifies transitions between window heat gain and loss. Examination of heat flux signals acquired with low-cost sensors first revealed the associated noise to be consistent but too great for control application. Representative noise characteristics were then used to construct noisy signals for simulations, allowing evaluation of noise mitigation by digital filtering methods and signal persistence requirements. Strikingly, shading controlled by the resulting output reduced window heat gain by 54–78% in six contrasting climates, comparable to or exceeding the performance of established approaches, indicating that heat flux sensing is now an outstanding candidate for shading control in passive cooling systems.

Thermal Conductivity of Detonation Nanodiamond Hydrogels and Hydrosols by Direct Heat Flux Measurements.

The methodology and results of thermal conductivity measurements by the heat-flow technique for the detonation nanodiamond suspension gels, sols, and powders of several brands in the range of nanoparticle concentrations of 2–100% w/w are discussed. The conditions of assessing the thermal conductivity of the fluids and gels (a FOX 50 heat-flow meter) with the reproducibility (relative standard deviation) of 1% are proposed. The maximum increase of 13% was recorded for the nanodiamond gels (140 mg mL−1 or 4% v/v) of the RDDM brand, at 0.687 ± 0.005 W m−1 K−1. The thermal conductivity of the nanodiamond powders is estimated as 0.26 ± 0.03 and 0.35 ± 0.04 W m−1 K−1 for the RUDDM and RDDM brands, respectively. The thermal conductivity for the aqueous pastes containing 26% v/v RUDDM is 0.85 ± 0.04 W m−1 K−1. The dignities, shortcomings, and limitations of this approach are discussed and compared with the determining of the thermal conductivity with photothermal-lens spectrometry.

Evaluation of thermal conductivity and esthetic quality of denture base resin composites with acrylic polymer and nanodiamonds

AbstractThe effect on the thermal conductivity and esthetic quality of acrylic denture base resins due to the addition of surface‐carboxylated nanodiamonds (NDs) was evaluated using direct heat flux measurement and spectrophotometric measurement, respectively. The addition of NDs to the denture resin improved its thermal conductivity, which was highly depended on the ND content. The thermal conductivity increased significantly upon the addition of a small amount of NDs, followed by a gradual decrease with increasing ND content. The influence of the ND content on the thermal conductivity of the ND‐impregnated resins was explained via experimental results and the theory of phonon scattering. Furthermore, changes in the appearance of the denture base resins caused by ND impregnation were observed to assess their esthetic qualities. The color changes in the resins with low ND contents were found to be minor. Therefore, the improved thermal conductivity and the retention of appearance of the resin can be simultaneously achieved by the addition of a small amount of NDs.

Software‐guided Microfluidic Reaction Calorimeter Based on Thermoelectric Modules

AbstractA software‐guided, continuous reaction calorimeter based on thermoelectric modules for direct heat flux measurements is presented. Sensors and actuators of the calorimeter's setup are implemented within a lab automation system, which enables the automated calibration of the heat flux sensors and investigations of chemical reactions through sequential function charts. Functionality of the calibration is shown by heat transfer experiments. Additionally, the calorimeter's performance is demonstrated by good agreement of conducted neutralization experiments with literature data.

On the Along‐Slope Heat Loss of the Boundary Current in the Eastern Arctic Ocean

AbstractThis study presents recent observations to quantify oceanic heat fluxes along the continental slope of the Eurasian part of the Arctic Ocean, in order to understand the dominant processes leading to the observed along‐track heat loss of the Arctic Boundary Current (ABC). We investigate the fate of warm Atlantic Water (AW) along the Arctic Ocean continental margin of the Siberian Seas based on 11 cross‐slope conductivity, temperature, depth transects and direct heat flux estimates from microstructure profiles obtained in summer 2018. The ABC loses on average (108) J m−2 per 100 km during its propagation along the Siberian shelves, corresponding to an average heat flux of 47 W m−2 out of the AW layer. The measured vertical heat flux on the upper AW interface of on average 10 W m−2 in the deep basin, and 3.7 W m−2 above the continental slope is larger than previously reported values. Still, these heat fluxes explain less than 20% of the observed heat loss within the boundary current. Heat fluxes are significantly increased in the turbulent near‐bottom layer, where AW intersects the continental slope, and at the lee side of a topographic irregularity. This indicates that mixing with ambient colder water along the continental margins is an important contribution to AW heat loss. Furthermore, the cold halocline layer receives approximately the same amount of heat due to upward mixing from the AW, compared to heat input from the summer‐warmed surface layer above. This underlines the importance of both surface warming and increased vertical mixing in a future ice‐free Arctic Ocean in summer.

Direct local heat flux measurement during water flow boiling in a rectangular minichannel using a MEMS heat flux sensor

Heat transfer characteristics of water flow boiling in a minichannel were investigated through the direct measurement of local heat flux using a fabricated MEMS heat flux sensor with a multi-layered structure. Local temperatures and heat fluxes in the water flow boiling were measured at a sampling frequency of 10 kHz in synchronism with the high-speed visualization of the boiling behavior. The measurement results revealed fundamental heat transfer processes, including thin liquid film evaporation, dry-out of the liquid film, transient heat conduction after dry patch rewetting, and single-phase forced convection. The thin liquid film evaporation indicated a high local heat flux that was well over 1 MW/m2 and provided a dominant contribution to the wall heat transfer. Furthermore, the post-rewetting transient heat conduction indicated a high heat flux that was higher than 1 MW/m2. However, it was insignificant in terms of the overall wall heat transfer at all the tested heat fluxes because of its short duration. The contribution of liquid-phase forced convection was also small, except at low vapor qualities.

Temperature-selective emitter

To achieve control of radiative emissivity of a material, we propose and demonstrate a vanadium dioxide (VO2)-based temperature-selective emitter. This emitter comprises layered VO2 and thin W-doped VO2 with decreased metal-insulator transition temperature. Because a metal–insulator–metal structure is realized only in the temperature range 46–61 °C, the emissivity enhanced only in this temperature range. We analytically calculated the temperature-dependent emissivity spectra and experimentally measured the temperature-dependent reflectance spectra and emissivity. Direct heat flux measurements of the fabricated device showed emissivities of 0.19, 0.45, and 0.24 for temperatures of &amp;lt;30 °C, 46–61 °C, and &amp;gt;71 °C, respectively. The emitter presented in this study contributes to the realization of the active control of thermal emission in various situations.

Surface Renewal Application for Estimating Evapotranspiration: A Review

The estimation of evapotranspiration (ET) is essential for meteorological modeling of surface exchange processes, as well as for the agricultural practice of irrigation management. Hitherto, a number of methods for estimation of ET at different temporal scales and climatic conditions are constantly under investigation and improvement. One of these methods is surface renewal (SR). Therefore, the premise of this review is to present recent developments and applications of SR for ET measurements. The SR method is based on estimating the turbulent exchange of sensible heat flux between plant canopy and atmosphere caused by the instantaneous replacement of air parcels in contact with the surface. Additional measurements of net radiation and soil heat flux facilitate extracting ET using the shortened energy balance equation. The challenge, however, is the calibration of SR results against direct sensible heat flux measurements. For the classical SR method, only air temperature measured at high frequency is required. In addition, a new model suggests that the SR method could be exempted from calibration by measuring additional micrometeorological variables. However, further improvement of the SR method is required to provide improved results in the future.

Experimental investigation and development of new correlation for flow boiling heat transfer in mini-channel

The flow boiling heat transfer of refrigerant R134a in horizontal multiport mini-channel was experimentally investigated in this paper. Two kinds of mini-channels made of aluminum alloy with the hydraulic diameter of 0.63 mm for 23 rectangular channels and 0.72 mm for 14 rectangular channels were studied. Electric heating film was adopted to provide direct uniform heat flux. The experimental results were obtained with the vapor quality between 0 and 1, the heat flux ranging from 10 to 60 kW/m2, the mass flux ranging from 128 to 560 kg/(m2.s) and the saturation pressure from 0.22 to 0.58 MPa. It was found that nucleate boiling dominates in the low vapor quality region and the heat transfer coefficients of mini-channel are higher than those in conventional channel. The heat transfer coefficients increase with the increasing of both heat flux and saturation pressure but have little dependence with the mass flux. In the high vapor quality region, connective boiling is the main contributor. Heat flux and saturation pressure have little influence on the heat transfer coefficients, but the influence of mass flux is obvious. There is a small increase in heat transfer coefficient with an increase of mass flux. Accordingly, a new correlation for flow boiling heat transfer in mini-channel of R134a was proposed based on Gungor and Winterton correlation. The mean relation deviation is −1.76% and the mean absolute relation deviation is 19.0% respectively. This new correlation is proved to have an obvious accuracy improvement in predicting the flow boiling heat transfer coefficient of R134a in mini-channel. The present work can provide a guidance to the design of mini-channel heat exchanger.

A process-level attribution of the annual cycle of surface temperature over the Maritime Continent

The annual cycle of the surface temperature over the Maritime Continent (MC) is characterized by two periods of rapid warming in March–April and September–October, respectively, and a period of rapid cooling in June–July. Based upon an analysis of energy balance within individual atmosphere–surface columns, the seasonal variations of surface temperature in the MC are partitioned into partial temperature changes associated with various radiative and non-radiative (dynamical) processes. The seasonal variations in direct solar forcing and surface latent heat flux show the largest positive contributions to the annual cycle of MC surface temperature while the changes in oceanic dynamics (including ocean heat content change) work against the temperature changes related to the annual cycle. The rapid warming in March–April is mainly a result of the changes in atmospheric quick processes and ocean–atmosphere coupling such as water vapor, surface latent heat flux, clouds, and atmospheric dynamics while the contributions from direct solar forcing and oceanic dynamics are negative. This feature is in contrast to that associated with the warming in September–October, which is driven mainly by the changes in solar forcing with a certain amount of contributions from water vapor and latent heat flux change. More contribution from atmospheric quick processes and ocean–atmosphere coupling in March–April coincides with the sudden northward movement of deep convection belt, while less contribution from these quick processes and coupling is accompanied with the convection belt slowly moving southward. The main contributors to the rapid cooling in June–July are the same as those to the rapid warming in March–April, and the cooling is also negatively contributed by direct solar forcing and oceanic dynamics. The changes in water vapor in all three periods contribute positively to the change in total temperature and they are associated with the change in the location of the center of large-scale moisture convergence during the onset and demise stages of the East Asian summer monsoon.

Kinetic investigation of the ion angular distribution in capacitive radio-frequency plasmas

One of the key parameters in the context of plasma assisted processing in semiconductor fabrication using capacitive radio-frequency plasmas is the ion flux distribution at the substrate. Whereas the ion energy distribution function determines the etching rate and selectivity, the ion angular distribution controls the etching profile. In this contribution, we reveal the effect of the ion flux and the sheath potential on the ion angular distribution and the direct ion heat flux at the bottom of etching profiles in geometrically symmetric plasma reactors. The ion angular distribution and the direct ion heat flux are calculated as a function of the sheath potential, the driving frequency, and the phase shift between the two distinct harmonics of the driving voltage of dual frequency discharges. For this task, self-consistent particle-in-cell simulations subject to Monte Carlo collision are carried out. The results from particle-in-cell simulations which are computationally very expensive are compared and verified with those from the novel ensemble-in-spacetime model. It is confirmed that increasing the voltage of the high-frequency component, the high-frequency component, and/or make a phase shift of π/2 between the dual frequency, narrow the ion angular distribution and increase the direct ion heat flux to the etching profile bottom. In all simulation cases, a correlation between the narrowing of the ion angular distribution and the increase of the sheath potential and the sheath ion flux is found.